Function classify_cache_run 

pub fn classify_cache_run(
    usage: &NormalizedCacheUsage,
    support: &PromptCacheSupport,
) -> CacheConformanceClassification

Classify one run from its normalized usage and the capability support verdict. Support status — never the presence/absence of a usage field — decides the zero-read bucket, so a missing field can’t masquerade as “unsupported”.